Composite metal articles

ABSTRACT

A method of forming a composite article having a first and a second metal components, and a resultant composite metal article, wherein a flux coating is applied over at least a substantially oxide-free bond surface of the first component, the first component with said flux coating is preheated and, with said first component positioned in a mould to fill a portion of a cavity of the mould, a melt for providing the second component is poured into the mould so as to flow over said bond surface; the first component being preheated to a first temperature and the melt being poured at a second temperature such that, on flowing over the bond surface, the melt displaces said flux coating and wets said bond surface, and that such initial temperature equilibration between said surface and the melt results in an interface temperature therebetween at least equal to the liquidus temperature of the melt, thereby resulting on solidification of the melt in attainment of a bond between the components.

The invention relates to composite metal articles. The inventionparticularly relates to articles of two different metals securely bondedtogether, with one metal protecting the other in a manner required for aparticular application.

A wide variety of procedures has been proposed for providing compositemetal articles to enable use of desirable properties of two dissimilarmetals. Thus, articles of a metal of low corrosion resistance frequentlyare protected by hard-facing or cladding with a wear or corrosionresistant metal such as stainless steel. Alternatively, tough butreadily machinable metals can be similarly protected by application of amaterial which provides in a composite article the required wearresistance. In the latter case, the tough metal supports and retains arelatively brittle abrasion resistant material which may fracture underimpact loading, while also enabling machining and fixing of thecomposite article in a manner possible only with difficulty for anarticle of abrasion resistant material alone.

Hardfacing by weld deposition of metal to provide a composite article,while widely used, is relatively slow, labour intensive, relativelycostly and subject to a number of practical limitations. However,recourse to hardfacing is necessary in many applications because of thelack of an economic and/or practical alternative. A variety ofalternative proposals is set out in U.K. patent specifications Nos.888404, 928928, 977207, 1053913, 1152370, 1247197 and 2044646 and inU.S. Pat. Nos. 3,279,006 and 3,342,564.

U.K. Pat. No. 888404 proposes a process for clad steel products, such asof mild or low alloy steel and a stainless steel, clad by casting a meltof one of the steels around a solid of the other steel. The solid othermetal is mechanically or chemically cleaned prior to the castingprocess, while casting is performed under a substantial vacuum. However,it is made clear that no complete bond is made merely by the castingprocess. The composite article thus has to be hot-rolled to weld the twosteels together; the bonding being effected by the hot rolling. Theprocess thus suffers from the disadvantages of having to be performedunder vacuum, a procedure not well suited to many production situations;while the need for hot rolling limits the choice of materials with whichthe process can be applied, as well as the form of the resultantcomposite article.

U.K. Pat. No. 928928 is concerned with liners for grinding mills, andpoints out the problems resulting from making the liner solely from anabrasion resistant material such as carbidic cast iron, eitherunalloyed, or an alloyed cast iron such as nickel-chromium white castiron. It thus proposes a composite liner of such material and a backingof a softer and tougher metal or alloy, produced by a double castingoperation in which a first metal is cast, and the second metal is castagainst the first metal. Evidently cognizant of the difficulty ofachieving a bond between a solid and a cast metal, and being unable witha brittle cast iron to have recourse to hot rolling to overcome thisdifficulty, U.K. Pat. No. 928928 teaches that the first metal, typicallythe carbidic cast iron, is only partially solidified when the secondmetal is cast against it.

U.K. Pat. No. 928928 recognises the adverse consequences of oxidation ofthe surface of the first metal against which the second metal is to becast. For this purpose, a chill mould is used to achieve rapid coolingof the first metal to its partially solidified condition. However, tofurther offset oxidation, a flux can be used to protect that surface;the flux being present in the mould before pouring the first metal oradded in liquid form with the first metal.

Due to the backing being cast in the proposal of U.K. Pat. No. 928928,its properties will be inferior to those of a wrought backing. Also, theneed for the first metal to be only partially solidified when castingthe second metal provides a substantial constraint. Thus, closetemperature control is imperative due to rapid cooling of the melt ofthe first metal and the need to cast the second metal while the first isonly partially solidified. Pouring of the second metal with the firststill too hot, that is, still containing liquid, will result in mixingof the metals, and loss of properties due to dilution; while, if thefirst metal is too cool, sound bonding is not likely. Also, the processnecessitates two melts available at the same time and at well-controlledtemperatures and, while some foundries will be able to meet this need,there remains the problem of coordinating pouring from the two ladlesnecessary. Additionally, there is the practical problem of feedingsolidification shrinkage in the cast first metal with metal of the samecomposition. In the disclosure of U.K. Pat. No. 928928, such shrinkagecan only be fed from the second metal, so that the first metalultimately will contain regions of dissimilar composition. Additionally,the process of U.K. Pat. No. 928928 necessitates the surface of thefirst metal being horizontal, with severe limitations on the range ofcomposite articles able to be produced. Further, the second metal has tobe fed horizontally over that surface to avoid excessive mixing of thetwo melts; while flow-rate of the second metal over that surface has tobe controlled so as to disturb the first metal as little as possible,for the same reason.

U.K. Pat. No. 977207 proposes a process for seamlessly clad products,such as pipes or rods, in which respective parts are of a soft steelsuch as stainless steel and a mild steel. In this process, a componentof one of those steels is heated under vacuum or a non-oxidizingatmosphere and, while maintaining such environment, it is plungedrapidly into a melt of the second steel. The temperature of heating ofthe component of the first steel is to be to a temperature such that, onbeing plunged into the melt of the second steel, its surface becomes asemi-molten or highly viscous melt such that, on cooling of the twosteels, they are welded together. The need for operation under a vacuumor a non-oxidizing atmosphere is a severe constraint, typicallynecessitating a sealed vessel in which the process is performed toexclude oxidation on heating the first component to near the meltingpoint of the second metal. Also, the process again is limited in therange of shapes or forms of composite articles able to be produced.Additionally, the process is not amenable to use where the two metalsdiffer significantly in melting point.

The severe disadvantages of operating with a non-oxidizing atmospherealso applies to the similar disclosures of U.K. Pat. Nos. 1053913 and1152370. These disclosures differ essentially in the composition oftheir respective wear resistant materials; U.K. Pat. No. 1053913proposes chromium-boron white cast irons containing molybdenum andvanadium, while U.K. Pat. No. 1152370 proposes nickel-boron cast ironscontaining molybdenum and vanadium. In each case the solid cast iron, inthe form of crushed pig and pellets, is sealed to prevent atmosphericoxidation in a housing in which it is to provide a lining and heatedtherein under an inert atmosphere so as to melt. The housing is spun tocentrifugally distribute the molten cast iron, and the housing and meltthereafter are cooled. In addition to the disadvantage of the need foran inert atmosphere, and spinning of the housing until the cast iron hassolidified, the disclosure of each of U.K. Pat. Nos. 1053913 and 1152370has other disadvantages. The housing, of necessity, must have a meltingpoint substantially above that of the cast iron, as the heating of thehousing has to be limited to a temperature below that at whichdistortion or deformation of the housing will occur, particularly whenspun. Additionally, the disclosure has severe limitations in relation tothe shape of the resultant composite article, given the reliance oncentrifugal distribution of the cast iron melt; while there is nodisclosure as to how as a practical matter the higher melting pointhousing can be provided with externally distributed cast iron.

U.K. Pat. No. 1247197 is similar overall to U.K. Pat. Nos. 1053913 and1152370. It differs principally in its use of eutectic Fe-C, plus highermelting point alloys, to form the cast iron.

U.S. Pat. Nos. 3,342,564 and 3,279,006 relate respectively to acomposite article and a method for its production in which a melt of onemetal is cast to fill a mould containing a solid second metal. Again, avacuum or non-oxidizing atmosphere is necessary, due to the second metalbeing preheated to an elevated temperature such that melting of itssurface occurs on casting of the first metal, and the need to protectagainst oxidation of the second metal.

Finally, U.K. Pat. No. 2044646 proposes hot welding together of a softsteel and a martensitic white cast iron. The welding together can beachieved by casting the white iron onto soft-steel plate, with thelatter possibly being preheated. Alternatively, the cast iron can becast first and, while still hot, the soft steel cast thereagainst.However, in the first of these alternatives, hot welding is likely onlyif surface melting of the soft-steel occurs, a situation not suggestedby the optional nature of possibly preheating the soft steel. Also,oxidation of the soft-steel occurs to such an extent that, even withmelting of the surface of the soft-steel, a sound bond between thesoft-steel and cast iron is hard to achieve. Similar considerationsapply in the second case, except that oxidation is of the cast ironduring its cooling. Indeed, it is only by mechanical interlockingresulting from perforations or the like in the one metal, against whichthe other is cast, that the two metals are likely to be adequatelysecured together. However, such interlocking obviates the advantage of asoft-steel backing in protecting the brittle cast iron under impactloading, as the interlocking gives rise to localized stressconcentration in the cast iron.

The present invention seeks to provide an improved composite metalarticle, and a processs for its production which is more amenable tosimple foundry practice and which enables a wider choice of metals.

The invention provides a method of forming a composite metal article,wherein a first metal component for the article is preheated and, withthe first component positioned in a mould cavity to fill a portion ofthe cavity, a melt for providing a second metal component is poured soas to flow into the cavity over a surface of the first component; thetemperature of said surface of the first component and the temperatureof the melt being controlled so as to achieve wetting of said surface bythe melt and attainment of a bond between the components onsolidification and cooling of the melt which is strengthened bydiffusion between the components and is substantially free of a fusionlayer of said surface of the first component.

The required bond substantially free of a fusion layer is achieved ifthe surface of the first component is wetted by the melt which is toform the second component. Such wetting of that surface is found tooccur if:

(a) a favourable surface energy relationship exists between the surfaceof the first component and the melt--a condition obtained if the surfaceis substantially free of oxide contamination but precluded by suchcontamination, and

(b) the first component has a relatively high melting point and itssurface, with the melt cast thereagainst, attains a sufficiently hightemperature, most preferably a temperature equal to or greater than theliquidus temperature of the melt.

The bond generally is sharply defined but typically exhibits some solidstate diffusion between the components. Also, while a fusion layerresulting from melting of the first layer substantially is avoided, thebond may be characterised by microdissolution, as distinct from melting,of the first component in the melt prior to solidification of thelatter. Additionally, some epitaxial growth from the surface of thefirst component can occur, although this has not been seen tocharacterize the bond to any visible extent.

Thus, it is found that the attainment of a sound bond by casting a meltof a metal against a solid component is dependent, inter alia, upon thetemperature prevailing at the surface of the solid component againstwhich the melt is cast, and also the absence of oxidation of thatsurface. In general, the prior art has endeavoured to protect againstoxidation by use of a vacuum or non-oxidizing atmosphere; a vacuumgenerally being preferred. However, as a practical matter, casting undervacuum is not well suited to industrial foundry practice andnecessitates expensive apparatus. Particularly in repetitive castingoperations, it also substantially increases production time. Similarcomments apply to casting under a non-oxidizing atmosphere since, toprovide adequate protection of the first component, casting under suchatmosphere must be performed in a closed vessel similar to thatnecessary when operating under vacuum. That is, particularly when thesolid first component is heated, as is necessary for a sound bond, theprecautions necessary to protect its surface against oxidation increasewith temperature and it is necessary that the melt for the secondcomponent be cast against the surface substantially in the absence ofoxide on the surface.

It is found that a sound bond is achieved if the surface of the firstcomponent is cleaned to remove any oxide film and then protected, untilthe melt for the second component is cast against it, by a film of asuitable flux. A variety of fluxes can be used, while these can beapplied in different ways. However, the flux most preferably is anactive flux in that it not only prevents oxidation of the surface of thefirst component, but also cleans that surface of any oxide contaminationremaining, or occurring, after cleaning of that surface. Suitable fluxesinclude Comweld Bronze Flux, which has a melting point of about 635° C.and contains 84% boric acid and 7% sodium metaborate, Liquid Air Formula305 Flux (650° C., 65% boric acid, 30% anhydrous borax) and CIG G.P.Silver Brazing Flux (485° C. and containing boric acid plus borates,fluorides and fluoborates). Less active fluxes, such as anhydrous borax(740° C.), which simply provide a protective film but do not removeexisting oxide contamination of the surface, can also be used providedthat such combination first is mechanically or chemically removed.

As indicated above, the temperature prevailing at the surface of thesolid component against which the melt is cast is an importantparameter. By this is meant the temperature at the interface between thecomponents on casting the melt. However, while important, this parameteris secondary to the need for that surface of the solid component to befree of oxide, since attainment of an otherwise sufficient interfacetemperature will not achieve a sound bond if that surface is oxidized.

The interface temperature attained is dependent on a number of factors.These include the temperature to which the solid component is preheated,the degree of superheating of the melt when cast, the area of thesurface of the solid component against which the melt is cast, and themass of the solid and cast components. Also, where the respective metalsof those components differ, further variables include the respectivethermal conductivity, specific heat and density of those metals.However, notwithstanding the complex inter-relationships arising fromthese parameters, it has been found that a satisfactory bond can beachieved when the solid component is preheated to a temperature of atleast about 350° C. The solid component preferably is preheated to atemperature of at least about 500° C.

It is highly preferred that the temperature to which the solid componentis preheated and the degree of superheating of the melt are such that,on casting the melt, an interface temperature equal to or in excess ofthe liquidus temperature for the melt is achieved. It is found that thesubstantially instantaneous interface temperature is not simply thearithmetic mean of the preheat and melt temperatures, weighted ifnecessary for differences in thermal conductivity, specific heat anddensity, as could be expected. Such arithmetic mean in fact results inerroneously low determination of substantially instantaneous interfacetemperature, since the calculation assumes that heat transfer from themelt to the solid component is solely by conduction. Calculation of theNusselt number for the melt shows that convection that transfer in themelt also is important and, when this is taken into account, it showsthe substantially instantaneous interface temperature may be up to about150° C. to 200° C. higher than the arithmetic mean of the preheattemperature of the solid component and the melt temperature.

The requirement that an interface temperature equal to or above theliquidus temperature of the melt be attained means that the inventionprincipally is applicable where the solid first component has a meltingrange commencing at a temperature at least equal to the liquidus of themelt to provide the second component. Also, it is to be borne in mindthat while reference is made in the preceding paragraph to thesubstantially instantaneous interface temperature, that reference is byway of example. That is, the required interface temperature need not beattained instantaneously, and may be briefly delayed such as due to atemperature gradient with the first component. It also should be notedthat the invention can be used where the melt to provide the secondcomponent is of substantially the same composition as the firstcomponent; the first and second components thus having substantially thesame melting range. In such case, it remains desirable that the surfaceof the first component against which the melt is cast still attains, oncasting of the melt, a temperature at least equal to the liquidustemperature of the melt, but that the body of the first component actsas a heat sink which quickly reduces that surface temperature beforesignificant fusion of the surface occurs. Similarly, the invention canbe applied where the first component has a melting range commencingbelow that of the material for the second component, provided such quickcooling can prevent significant surface fusion of the first component;although such lower melting range first component is not preferred.

Attainment of a sufficient interface temperature is achieved by abalance between preheating of the first component, and the extent ofsuperheating of the melt to provide the second component. The preheatingpreferably is to a temperature in excess of 350° C., more preferably toat least 500° C. The melt preferably is superheated to a temperature ofat least 200° C., most preferably at least 250° C., above its liquidustemperature. However, in the case of aluminium bronzes such ashereinafter designated which are highly prone to oxidation, it can bedesirable to drop these limits to 100° C. and 150° C. respectively, witha corresponding increase in preheating of the substrate.

The use of a flux and attainment of a sufficient interface temperatureenables a sound bond to be achieved between similar metals and alsobetween dissimilar metals. We have found that these factors enable suchbond to be achieved in casting a stainless steel against a mild steel,or an alloy steel such as a stainless steel. A sound bond also similarlyis round to be achieved in casting a cast iron, for example, a whitecast iron such as a chromium white cast iron, against a mild steel, analloy steel such as a stainless steel, or cast iron such as a white castiron. Additionally, cobalt-base alloys similarly can be cast against amild steel or an alloy steel to achieve a sound bond therebetween.Moreover, similar results can be achieved in casting nickel alloys, suchas low melting point nickel-boron alloys, and aluminium bronzes againstmild steel or alloy steels.

Stainless steels with which excellent results can be achieved, either asthe solid first component or the cast second component, include thosesuch as austenitic grades equivalent to AISI 316 or AS 2074-H6A, having0.08 wt.% maximum carbon, 18 to 21 wt.% chromium, 10 to 12 wt.% nickeland 2 to 3 wt.% molybdenum, the balance substantially being iron. AISI304 stainless steel, with 0.08 wt.% maximum carbon, 18 to 21 wt.%chromium, 8 to 11 wt.% nickel, and the balance substantially iron, alsocan be used.

Suitable cobalt base alloys include those of compositions typified by(Co,Cr)₇ C₃ carbides in an eutectic structure and a work hardenablematrix, such as compositions comprising 28 to 31 wt.% chromium, 3.5 to5.5 wt.% tungsten, 3.0 wt.% maximum iron, 3.0 wt.% maximum nickel, 2.0wt.% maximum manganese, 2.0 wt.% maximum silicon, 1.5 wt.% maximummolybdenum, 0.9 to 1.4 wt.% carbon and the balance substantially cobalt.A cobalt base alloy having the nominal composition 29 wt.% chromium, 6.3wt.% tungsten, 2.9 wt.% iron, 9.0 wt.% nickel, 1.0 wt.% carbon and thebalance substantially cobalt, also has been found to be suitable.

Cast irons used as the second component include chromium white irons, ofhypo- or hyper-eutectic composition. For these the carbon content canrange from about 2.0 to 5.0 wt.% while the chromium content can besubstantially in excess of chromium additions used to decreasegraphitization in cast iron. The chromium content preferably is inexcess of 14 wt.% and may be as high as from 25 to 30 wt.%. Conventionalalloying elements normally used in chromium white iron can be present inthe component of that material. Particular chromium white irons found tobe suitable in the present invention include:

(a) AS 2027 grade Cr-15, Mo-3, cast iron having 2.4 to 3.6 wt.% carbon,0.5 to 1.5 wt.% manganese, 1.0 wt.% maximum silicon, 14 to 17 wt.%chromium and 1.5 to 3.5 wt.% molybdenum, the balance apart fromincidental impurities being iron.

(b) AS 2027 grade Cr-27 cast iron having 2.3 to 3.0 wt.% carbon, 0.5 to1.5 wt.% manganese, 1.0 wt.% maximum silicon, 23 to 30 wt.% chromium,and 1.5 wt.% maximum molybdenum, the balance apart from incidentalimpurities being iron.

(c) austenitic chromium carbide iron having 2.5 to 4.5 wt.% carbon, 2.5to 3.5 wt.% manganese, 1.0 wt.% maximum silicon, 25 to 29 wt.% chromium,and 0.5 to 1.5 wt.% molybdenum, the balance apart from incidentalimpurities being iron.

(d) complex chromium carbide iron having 4.0 to 5.0 wt.% carbon, 1.0wt.% maximum manganese, 0.5 to 1.5 wt.% silicon, 18 to 25 wt.% chromium,5.0 to 7.0 wt.% molybdenum, 0.5 to 1.5 wt.% vanadium, 5.0 to 10.0 wt.%niobium, and 1.0 to 5.0 wt.% tungsten, the balance apart from incidentalimpurities being iron.

(e) complex chromium carbide iron having 3.5 to 4.5 wt.% carbon, 1.0wt.% maximum manganese, 0.5 to 1.5 wt.% silicon, 23 to 30 wt.% chromium,0.7 to 1.1 wt.% molybdenum, 0.3 to 0.5 wt.% vanadium, 7.0 to 9.0 wt.%niobium, and 0.2 to 0.5 wt.% nickel, the balance apart from incidentalimpurities being iron.

Suitable nickel alloys include nickel-boron alloys conventionallyapplied by hard-facing and characterized by chromium borides andchromium carbides in a relatively low melting point matrix. Particularlypreferred compositions are those substantially of eutectic compositionand having 11 to 16 wt.% chromium, 3 to 6 wt.% silicon, 2 to 5 wt.%boron, 0.5 to 1.5 wt.% carbon and optionally 3 to 7 wt.% iron thebalance, apart from incidental impurities being nickel. Exemplarycompositions are:

(a) 77 wt.% nickel, 14 wt.% chromium, 4.0 wt.% silicon; 3.5 wt.% boronand 1.0 wt.% carbon, plus incidental impurities; and

(b) 13.5 wt.% chromium, 4.7 wt.% iron, 4.25 wt.% silicon, 3.0 wt.%boron, 0.75 wt.% carbon and, apart from incidental impurities, a balanceof nickel.

Aluminium bronze compositions suitable for use in the invention varyextensively but, excluding impurities, are typified by:

(a) 86 wt.% minimum copper, 8.5 to 9.5 wt.% aluminium and 2.5 to 4.0wt.% iron (UNS No. C95200);

(b) 86 wt.% minimum copper, 9.0 to 11.0 wt.% aluminium, and 0.8 to 1.5wt.% iron (UNS No. C95300);

(c) 83 wt.% minimum copper, 10.0 to 11.5 wt.% aluminium, 3.0 to 5.0 wt.%iron, 2.5 wt.% maximum nickel (plus any cobalt), and 0.5 wt.% maximummanganese (UNS No. C95400);

(d) 78 wt.% minimum copper, 10.0 to 11.5 wt.% aluminium, 3.0 to 5.0 wt.%iron, 3.0 to 5.5 wt.% nickel (plus any cobalt), and 3.5 wt.% maximummanganese (UNS No. C95500);

(e) 71 wt.% minimum copper, 7.0 to 8.5 wt.% aluminium, 2.0 to 4.0 wt.%iron, 11.0 to 14.0 wt.% manganese, 1.5 to 3.0 wt.% nickel, 0.10 wt.%maximum silicon, and 0.03 wt.% maximum lead

(f) 79 wt.% minimum copper, 8.5 to 9.5 wt.% aluminium, 3.5 to 4.5 wt.%iron, 0.8 to 1.5 wt.% manganese, 0.10 wt.% maximum silicon and 0.03 wt.%maximum lead (UNS No. C95800); and

(g) 12.5 to 13.5 wt.% aluminium, 3.5 to 5.0 wt.% iron, 2.0 wt.% maximummanganese, 0.5 wt.% maximum other elements, balance substantially copper(UNS No. C62500).

The aluminium bronze alloys exhibit poor castability, as is appreciated.A problem with their use in the present invention is the pronouncedtendency for their melts to oxidize, and this can complicate their usein the invention as in other applications. However, protecting the meltagainst oxidation, such as by melting under a flux cover, enables thesealloys also to be cast against and securely bonded to a solid firstcomponent, such as a mild steel substrate. However, because of thetendency for the melt to oxidize, it can be advantageous to limit theextent of superheating of the melt and to achieve the required firstcomponent/melt interface temperature by increasing the temperature towhich the first component is preheated.

The specifically itemised castable metals suitable for use in theinvention as the second component will be recognised as surfacingmaterials conventionally applied by hardfacing by weld deposition.Typically, such metals are applied to provide wear resistant facings.However, in the case of stainless steels, which can provide abrasionresistance at low or medium temperatures, the purpose of its use in acomposite article may be in part or wholly to achieve corrosionresistance for the other component of the article. Thus, whileprincipally concerned with composite articles having abrasion resistanceby appropriate selection of the metal of one component, the inventionalso is concerned with articles for use in environments other than thosein which abrasion resistance is required. Also, as indicated by theability to cast for example a cast iron against a cast iron, thecomposite article of the invention can be applied to rebuilding a wornor damaged part of an article; the first and second components in thatcase being of substantially the same or similar composition if required.In such rebuilding, the worn or damaged part of an article can bemachined, if required, to provide a more regular surface thereof againstwhich a melt of rebuilding metal is to be cast. However, such machiningmay not be necessary for a sound bond to be achieved, provided that anoxide-free surface is available against which to cast the melt.

The solid first component may be preheated in the mould or prior tobeing placed in the mould while the type of mould used can vary with thenature of the preheating. When heated in the mould, the preheating maybe by induction coils, or by flame heating. When heated prior to beingplaced in the mould, resistance, induction or flame heating can be usedor, alternatively, the solid first component can be preheated in amuffle or an induction furnace. What is important, in each case, is thatat least the surface of that component against which the melt for thesecond component is to be cast is thoroughly cleaned mechanically and/orchemically and protected, prior to preheating to a temperature at whichre-oxidation will occur, by a suitable flux. Normally, in such cases,the flux is applied as a slurry, such as by the flux being painted on atleast that surface of the solid first component. Alternatively, the fluxcan be sprinkled on the surface in powder form; provided, wherepreheating then is to be by a flame, the surface has been partiallyheated to a temperature at which the flux becomes tacky. Particularlywhere the surface of the first component against which the melt is to becast is of complex form, the flux alternatively can be applied bydipping the first component into a bath of molten flux. In each of thesemethods of applying the flux, the first component can be stored, oncecoated with the flux, until required for preheating. Alternatively, thecomponent may be preheated immediately after the flux is applied.

Where the flux is applied by dipping the solid first component in a bathof molten flux, a variant on the above described methods of preheatingcan be adopted. In this, the preheating can be effected at least in partby the solid first component being soaked in the bath of molten fluxuntil it attains a sufficient temperature, which may be below,substantially at, or above the required preheat temperature. Thecomponent then can be transferred to the mould and, after furtherinduction or flame heating or after being allowed to cool to therequired preheat temperature, the melt to provide the second componentis cast thereagainst.

Where preheating of the solid first component is at least in part byflame heating, that component may be positioned in a mould defining afiring port enabling a heating flame to extend into the mould cavity andover that component; the flame preheating the component and also heatingthe mould. While not essential, a reducing flame can be used to maintainin the mould a reducing atmosphere so as to further preclude oxidationof the surface of the first component. The flame may be provided by aburner adjacent to the firing port for generating the reducing flame.

The mould for use in flame heating may be constructed in portions whichare separable. The portions may be spaced by opposed side walls and, atone end of those walls, the firing port can be defined, with an outletport for exhausting combustion gases from the flame being defined at theother ends of the side walls. The side walls may be separable from themould portions or each may be integral with the same or a respectivemould portion. Preferably, an inlet duct is provided at the firing portfor guiding the flame into the interior of the mould. Where the firstcomponent has an extensive surface over which the melt is to be cast,such as a major face of a flat plate substrate, the width of the firingport in a direction parallel to that surface may be substantially equalto the dimension of the substrate surface in that direction. The ductmay have opposed side walls which diverge toward the firing port tocause the reducing flame to fan out to a width extending oversubstantially the full surface of the substrate to which the melt is tobe cast. Also, the duct may have top and bottom walls which convergetoward the firing port to assist in attaining such flame width. The ductmay be separable from the mould, integral with one mould portion orlongitudinally separable with a part thereof integral with each mouldportion.

The flame heating may be maintained until completion of casting of themelt. After pouring the melt and before the latter has solidified, theburner may be adjusted to give a hotter, slightly lean flame.Solidification of the top surface of the melt can be delayed by suchlean flame, so that the melt solidifies preferentially from themelt/first component interface, rather than simultaneously from thatinterface and top surface. Such solidification also can minimise voidformation due to shrinkage in the unfed cast metal.

In such flame preheating, the pouring arrangement most conveniently issuch as to rapidly distribute the melt over all parts of the surface ofthe first component on which it is to be cast and to maximise turbulencein the melt. Such rapid distribution and turbulence promotes heattransfer and a high, uniform temperature at the interface between thepoured melt and the surface first component. Rapid distribution andturbulence also facilitates breaking-up and removal of any oxide film onthe melt. It also would remove any residual oxide film of that surface,although reliance on this action without prior cleaning and use of aflux produces a quite inferior bond.

Rapid distribution of the melt over the substrate surface of the firstcomponent and turbulence in the melt can be generated by a mould havinga pouring basin into which the melt is received, and from which the meltflows via a plurality of sprues of which the outlets are spaced overthat surface. This arrangement functions to evenly and simultaneouslypour the melt onto all areas of the surface; thereby reducing thedistance the melt has to flow and aiding in achieving a high and uniformtemperature at the melt-first component interface. The arrangement alsoincreases turbulence in the melt over, and facilitates wetting of, thatsurface.

One advantage of a reducing flame in such preheating of the firstcomponent is that it offsets any tendency for oxidation of the meltresulting from its rapid distribution and turbulence. Also, suchturbulence can cause erosion, by localized macrodissolution of metal ofthe first component, at points of impingement of the melt with thesurface of that component. It therefore can be beneficial to use anarrangement for pouring the melt which establishes substantiallynon-turbulent, progressive mould filling. In one such arrangement, theinvention uses a mould having a horizontally extending gate which causesthe melt to enter a mould cavity in a plane substantially parallel to,and slightly above, the surface of the first component on which the meltis to be cast. This enables the melt to progress in substantiallynon-turbulent flow across the surface, with minimum division of theflow, thereby inhibiting oxidation of the melt. Thus, the exposure offresh, non-oxidized metal of the melt to an oxidizing environment isminimised.

The placement of the gate most conveniently is such that the initialmelt which enters the mould flow across the surface of the pre-heatedfirst component, further heating that surface. Subsequent incomingliquid metal displaces the initial metal which entered the mould cavity,thereby ensuring that maximum heat is imparted to the surface beforesolidification commences. Just prior to pouring, the mould cavity may beclosed with a cope-half mould, with the molten metal being run into thecavity through a vertical down sprue and horizontal runner system. Forsmall castings, this system permits several castings to be made in thesame moulding box from a single vertical down-sprue feeding intoseparate runners for each casting. Such casting practice can be used toproduce a bond interface on a horizontal, inclined or even vertical,surface of the first component.

In such arrangement providing substantially non-turbulent flow of themelt in the mould, flame heating again can be used. However, in thisinstance, it is necessary to position the first component (which mayhave been partially preheated) in the drag portion of the mould and,before positioning the cope portion of the mould, to effect flameheating from above. As an alternative, the mould can be fully assembledand preheating effected or completed therein by induction heating.

Where flame heating is used, it is preferred that the flux be applied bydipping in a melt of the flux or by painting on a slurry of the flux.If, as an alternative, it is required to apply the flux as a powder, itis preferable that the first component be slightly heated to about 150°to 200° C., such as in a muffle furnace, so that the flux becomes tackyand is not blown from the surface of the first component by the heatingflame.

When the flux is applied by dipping the first component into a bath ofmolten flux, the flux is applied at least over the surface of thatcomponent against which the melt is to be cast. Preferably, thecomponent is immersed in the bath so as to be fully coated with flux andalso at least partially preheated in that bath. Once a flux coating isprovided, the first component then is positioned in a mould and a meltto provide the second component poured into the mould so that the meltflows over the surface of the first component. Preferably the firstcomponent is suspended in the bath of molten flux until its temperatureexceeds the melting point of the flux. The component is then withdrawnfrom the flux bath with a coating of a thin, adherent layer of the fluxthereon. The melt displaces the thin flux coating, remelting the latterif necessary, thereby exposing the clean surface of the first componentso that wetting and bonding take place. Clearly, the flux employed musthave a melting point which is sufficiently low to permit quick remeltingof the flux, if frozen at the time the melt is poured into the mould. Atthe same time the molten flux must be able to withstand temperaturessufficiently high that the steel substrate can be adequately preheated.A sufficient temperature can be achieved with several fluxes duringsuspension, or dipping, of the first component in the bath of moltenflux. However, where the temperature of the flux bath is insufficientfor this, or where the heat loss from the first component betweenforming the flux coating and pouring the melt is too great, the firstcomponent can be further preheated in the mould, such as by induction orflame heating.

BRIEF DESCRIPTION OF THE DRAWING

In order that the invention may more readily be understood, descriptionnow is directed to the accompanying drawings, in which:

FIG. 1 shows, in vertical section, a furnace suitable for use in a firstform of the invention;

FIG. 2 is a horizontal section, taken on line II--II of FIG. 1;

FIG. 3 is a perspective view of a pouring mould pattern suitable formaking a mould component of a furnace as in FIGS. 1 and 2;

FIG. 4 shows a flow chart depicting the manufacture of composite metalarticles in a second form of the invention; and

FIG. 5 shows a flow chart depicting a third form of the invention.

With reference to FIGS. 1 and 2, mould 10, formed from a bonded sandmixture, has a lower mould portion 12 in which is positioned a ductilefirst component or substrate 14 on which a wear-resistant component isto be cast. A layer 16 of ceramic fibre insulating material insulatesthe underside of substrate 14 from the mould portion 12, while a layer18 of such material lines the side walls of portion 12 around the abovesubstrate 14. Mould 10 also has an upper portion 20, spaced aboveportion 12 by opposed bricks 22. The spacing provided between portions12,20 by bricks 22 is such as to define a transverse passage 24 throughmould 10. Across one end of passage 24, the mould is provided with aninlet duct 26; the junction of the latter with passage 24 defining afiring port 28. A burner 30, operable for example on gas or oil, ispositioned adjacent to the outer end of duct 26 for generating a flamefor preheating substrate 14 and mould portions 12,20.

Duct 26 has sidewalls 32 which diverge from the outer end to firing port28. This arrangement causes the flame of burner 30 to fan outhorizontally across substantially the full width of port 28 and, withinmould 10, to pass through passage 24 over substantially the entire uppersurface of substrate 14. Upper and lower walls 34,35 converge to port28, and so assist in attaining such flame width in mould 10. The flamemost conveniently extends through the end of passage 24 remote from port28; with combustion gases also discharging from that remote end.

Upper portion 20 of the mould has a section 36 defining a pouring basin37 into which is received the melt of wear-resistant metal to be cast onthe upper surface of substrate 14. From basin 37, the melt is able toflow under gravity through throat 38, along runners 39, and through theseveral sprues 40 in portion 20. The lower ends of sprues 40 aredistributed horizontally, such that the melt is poured evenly andsimultaneously onto all areas of the upper surface of substrate 14.

FIG. 3 shows a mould pattern for use in producing the upper portion 20of a mould similar to that of FIGS. 1 and 2. In FIG. 3 correspondingparts are shown by the same numeral primed.

Castings made in a mould as shown in FIGS. 1 and 2 include steelsubstrates measuring 300 mm×300 mm and 10 mm thick. The steel plateswere inserted in the lower mould portion with insulation under andaround the plates as described earlier. The moulds were levelled, fluxwas sprinkled on the steel to cover its upper surface, the mould builtup in the manner discussed, and the mould was initially gently heated tomake the flux tacky and adhere to the surface. Two sizes of castingswere made using a high chromium white cast iron, one type had 40 mmoverlay on 10 mm steel plate, the other had 20 mm on 10 mm.

For the 4:1 ratio castings, the substrate was preheated by means of theburner generating a reducing flame in the mould, and 30 kg of highchromium white iron was poured at a temperature of approximately 1600°C. into the pouring basin. The iron surface was kept liquid for about 8minutes and the burner was then turned off. A thermocouple against thebottom surface of the substrate reached a temperature of 1250° C.approximately 2 mins. after pouring. Ultra-sonic measurement indicated100% bonding, which was subsequently confirmed by surface grinding ofthe edges and of a diagonal cut through the casting, as well asextraction of 50 mm diameter cores by electro-discharge machining. Thebond was free of any fusion layer due to melting of the steel.

For the 2:1 ratio castings, the substrate was preheated and 15 kg of theiron was poured at a temperature of about 1600° C. The white ironsurface could not be kept liquid as long as with the 4:1 ratio castings,but was liquid for about 5 minutes. The thermocouple against the bottomof the plate reached 1115° C. approximately 3 minutes after pouring. Forthis size casting sound bonding over the full interface between thesubstrate and cast metal again is achieved.

In addition to the castings described above, a number of furthercastings were made on 200 mm×50 mm×10 mm steel substrates. The mostsuitable pouring mould in this case was found to be in the shape of afunnel with a long narrow slot at the bottom. The slot extended for thefull length of the substrate and was narrow enough for the liquid ironto issue from its full length simultaneously. With a preheat of 350° C.and a liquid iron pour temperature of 1570° C., bonding was achievedover more than 95% of the total area. By increasing the preheattemperature, bonding over 100% of the area can readily be achieved withthis size of substrate.

The castings described have been shown to give complete bonding on 300mm×300 mm×10 mm test plates of mild steel with white iron to steelratios of 4:1 to 2:1. Higher and lower ratios are possible; the lowerratios depending in part on substrate thickness and the rate of heatloss from the metal for optimum bonding.

Inherent in the invention is a high degree of freedom with respect tothe geometrical shape of the substrate and the finished article. Theinvention has significant advantages compared to other methods in thatit enables the direct casting of hard, wear-resistant metals, such ashigh chromium white iron, onto ductile steel substrates. The finishedarticle can combine the wall documented wearing qualities of for examplewhite iron with the good mechanical strength and toughness, machiningproperties and weldability of low carbon steel. The direct metallurgicalbond between the white iron and the steel results in very high bondstrength. The invention is especially suitable for producing hardfacinglayers of thickness exceeding those which may be conveniently laid downby welding processes.

The temperature to which the substrate is preheated can varyconsiderably. The temperature is limited by the need to preventoxidation, the melting point of the material of the substrate, the needto minimise grain growth, and the type of flux. Within these limits, ahigh preheat temperature is advantageous. The minimum preheattemperature will depend on the thickness ratio of cast component tosubstrate, and on the size and shape of the components. For theabove-mentioned 4:1 castings, a preheat temperature of 500° C. was foundto be just sufficient; while for the 2:1 castings, a minimum preheat of600° C. was found to be necessary.

An important parameter is the temperature at the interface between thecast liquid and the substrate. This enables a lowering of melttemperature with a corresponding increase in substrate preheattemperature, and vice versa. However, it is preferable for the melt tobe superheated sufficiently to allow any flux and any dislodged scale torise to the surface of the cast melt, and to attain the requiredinterface temperature for a satisfactory bond between the substrate andcast component. For all casting alloys, with the exception of aluminiumbronzes discussed herein, superheating by at least 200° C. above theliquidus temperature is preferred, most preferable at least 250° C.above that temperature, in order to achieve the required interfacetemperature on casting.

Particularly with the flux provided over the substrate surface on whichthe melt is to be cast, the reducing flame need provide only a mildlyreducing atmosphere over that surface during preheating. For suchatmosphere, a flame provided by an air deficiency of between 5% and 10%can be used.

With reference to FIG. 4, there is shown at A an underside view of thecope portion 50 of mould 52, and the top plan view of drag portion 54thereof. In each of several mould cavities 56, there is a respectivechamfered substrate 58, of which the upper surface of each has beenpainted with a flux slurry. As shown at B, substrates 58 are preheatedby flame from above, prior to positioning cope portion 50, using areflector 60 to facilitate preheating. As shown at C, cope portion 50then is positioned and a melt to be cast against the upper surface ofeach substrate is poured into the mould via cope opening 62. The meltflows horizontally via gates 64, to each cavity 56, and flows along eachsubstrate 58 across the full width of each. As indicated at D, theresultant composite articles 66 are knocked-out, and thereafter dressedin the normal manner.

Operation as depicted in FIG. 4 has been used to produce various sizesof hammer tips for use in sugar cane shredder hammer mills. The hammertips were made with mild steel substrates and a facing bonded thereto ofhigh chromium white cast iron. Dimensions of hammer tips produced havebeen as follows:

    ______________________________________    Substrate dimensions (mm)                     Cast overlay thickness (mm)    ______________________________________    80 × 90 × 25 (thick)                     25    90 × 90 × 25 (thick)                     20    76 × 50 × 20 (thick)                     18    ______________________________________

Risers have been employed in producing the hammer tips to ensure fullysound castings were produced. In these types of hammer tip, substantialchamfers have been machined into the substrates prior to pouring, inorder to permit the production of hammer tips with a more completecoverage of wear-resistant alloy on the working face than has hithertobeen possible with brazed composites. These hammer tips have also usedpre-machined substrates, wherein drilled and tapped holes required forsubsequent fixing of the hammer tip to the hammer head have been formedprior to production of the composite. The threaded holes have beenprotected with threaded metal inserts during the casting operation. Theflexibility of being able to use pre-machined bases in this way hasovercome the problems associated with drilling and tapping blind holesin an already bonded composite.

The hammer tips were found to be characterized by a sound diffusionbond, using casting temperatures comparable to those indicated withreference to FIGS. 1 to 3.

The bonds were diffusion bonds exhibiting no fusion layer due to meltingof the substrate surfaces.

With reference to FIG. 5, there is shown at A a furnace 70 providing abath of molten flux 72 in which is immersed a tubular steel component74. The latter is preheated to a required temperature in flux 70. Asindicated at B and C, heated component 74 coated with flux, is withdrawnfrom furnace 70 and, after draining excess flux, component 74 is loweredinto the drag half 76 of a mould and the cope half 78 of the latter ispositioned. In the arrangement illustrated, the mould includes a core 80which extends axially through component 74, to leave an annular cavity82 between core 80 and the inner surface of component 74. With cope half78 positioned as shown at D, a melt of superheated metal is cast as atE, via cope opening 84, to fill cavity 82.

Trials with the above described Liquid Air flux (m.p. 650° C.) have beencarried out in a procedure essentially as described with reference toFIG. 5, using steel substrates comprising:

(a) 200 mm long×50 mm wide×10 mm thick, for which bonding has beenproduced with cast overlay thicknesses of 40 mm, 30 mm and 20 mm (i.e.4:1, 3:1 and 2:1 casting ratios); and

(b) 80 mm square×25 mm thick, for which good bonding has been producedwith a cast overlay thickness of 25 mm (i.e. 1:1 casting ratio).

It has been found that the flux layer which adheres to the substrateupon its withdrawal from the molten flux bath is relatively thick, andthat mechanical scraping away of the majority of this adherent flux toleave only a very thin layer produced a better bond. A lower meltingpoint flux can be used and has the advantages of being more fluid at therequired working temperature, thereby draining better upon withdrawal ofthe substrate as well as being more readily remelted during casting.However, in the latter regard, it should be noted that it is notnecessary that the flux freezes between removal of the substrate fromthe bath and casting the melt or the application of flame or otherpreheating. Also, use of a lower melting point flux facilitatesproduction of even smaller casting ratio articles than described herein.

While the articles described herein are of planar form, it should benoted that the invention can be used to provide articles of a variety offorms. Thus, the invention can be used in the production of, forexample, cylindrical articles having a wear-resistant material cast onthe internal and/or external surface thereof, curved elbows, T-piecesand the like. Representative further composite articles furtherexemplifying the flexibility and range of possibilities with the presentinvention are set out in the following table, in which:

Method I designates manufacture in accordance with the proceduresdescribed with reference to FIGS. 1 to 3, and

Methods II and III designate manufacture in accordance with FIGS. 4 and5, respectively.

                                      TABLE    __________________________________________________________________________    Substrate Component                       Cast Component      Method    __________________________________________________________________________    A. Alloy White Cast Iron    1. 200 × 50 × 10 mild steel                       Each of 40, 30, 20 and 10 mm                                           Each of I, flame preheating       plates          on substrate main faces.                                           and III, flux bath preheating.    2. 300 × 300 × 20 mm thick                       Each of 40 and 20 mm on                                           I, flame preheating.       steel plates    substrate main faces.    3. 900 × 75 × 50 mm steel bar                       50 mm thickness on main face                                           I, flame preheating articles                       (heat/abrasion resistant alloy                                           for use as sinter plant                       complex Cr--carbide iron).                                           griller bars.    4. Steel plate of: Cast on substrate mainfaces                                           Both I and II, flame pre-       (a) 80 × 70 × 25 mm                       25 mm               heating - articles for use       (b) 90 × 80 × 25 mm                       25 mm               as hammer tips in sugar cane       (c) 76 × 50 × 20 mm                       20 mm               shredder.       (d) 90 ×  90 × 20 mm                       25 mm    5. Round steel bar of:                       Cast on cylindrical cladding       (a) 40 mm diameter                       30 mm wall thickness       (b) 50 mm diameter                       25 mm wall thickness                                           III, flux bath preheating.       (c) 60 mm diameter                       20 mm wall thickness       (d) 70 mm diameter                       15 mm wall thickness    6. Hollow steel pipes of:                       Cast to provide:       (a) 100 mm outside diameter,                       Internal claddings of each of       and 10 mm wall thickness.                       15 mm and 19 mm.       (b) 75 mm outside diameter,                       External cladding of 12.5 mm       and 10 mm wall thickness.                       with simultaneous internal                       claddings of each of 3.5 and                                           III, flux bath heating.                       7.5 mm thicknesses.       (c) 90° pipe bend of 75 mm                       Internal cladding of 7 to 10 mm       outside diameter, 5 mm                       thickness.       wall thickness and 63 mm       centreline radius of curvature.    7. AISI 304 stainless steel,                       Cast 25 mm on main substrate                                           II, induction preheating.       90 × 90 × 10 mm thick                       faces.    8. Composite substrate 90 ×                       Cast 25 mm on main substrate                                           II, induction preheating.       90 × 25 mm with 15 mm                       white iron overlay surface.       thick base of mild steel       and 10 mm thick white       iron overlay    B. Stainless Steel    9. (a) 90 × 90 × 10 mm thick                       AISI 316 stainless steel cast                                           II, induction preheating.       mild steel      25 mm on main substrate surface.       (b) 90 × 90 × 70 mm thick                       Cast on main face 70 mm thickness.                                           II, induction preheating                                           - plate  and III, flux bath                                           preheating.    C. Cobalt Base Alloy    10.       90 × 90 × 10 mm thick                       Cast on main substrate face                                           II, induction preheating.       mild steel      25 mm thickness.    D. Aluminium Bronze Alloy       90 × 90 × 10 mm thick                       Cast 25 mm on substrate main                                           II, with flame preheating       mild steel plates                       faces.              and II with induction                                           preheating.    E. Nickel Alloy       90 × 90 × 10 mm thick                       Cast 25 mm on substrate main                                           II, with induction       mild steel plate                       faces.              preheating.    __________________________________________________________________________

With each of the examples detailed in the table, sound bonds wereachieved in each case. It was found that attainment of a sound bond wasrelatively insensitive to the choice of flux, or the method ofpreheating, in any of those cases. Generally, preheating of thesubstrate component was to a temperature of about 800° C., with the meltpoured at a temperature of about 1600° C. for all alloys exceptaluminium bronze. The above-mentioned CIG Silver Brazing Flux and LiquidAir 305 Flux both were found to be highly suitable, particularly inmethod III.

The melt used in Example 12 was 14.7 wt.% aluminium, 4.3 wt.% iron, 1.6wt.% manganese, the balance, apart from other elements at 0.5 wt.%maximum, being copper. As with other aluminium bronze compositionsdetailed herein, this melt exhibited a tendency to oxidation, andprecautions are necessary to prevent this. To the extent that thisdifficulty could be overcome, sound bonding at clean interface surfacesresults. The melt liquidus is approximately 1050° C. and the melt waspoured at 1350° C. with the substrate preheated to about 800° C. Theproblem of melt oxidation can be reduced by lowering the meltsuperheating, with a corresponding increase in substrate preheatingand/or use of a flux cover for the melt.

The melt used in Example 13 had a composition of 13.5 wt.% chromium, 4.7wt.% iron, 4.25 wt.% silicon, 3.0 wt.% boron, 0.75 wt.% carbon and thebalance substantially nickel. This melt had a liquidus temperature ofapproximately 1100° C., and was poured at approximately 1600° C. withthe substrate preheated to approximately 800° C.

The bond achieved with the present invention was found to be of goodstrength. This is illustrated for a composite article comprising AISI316 stainless steel cast against and bonded to mild steel. For sucharticle, bond strengths of about 440 MPa were obtained with testspecimens machined to have a minimum cross-section at the bond zone.Also with such article, an ultimate tensile strength of about 420 MPawas obtained in a testpiece with 56 mm parallel length, with the bondabout halfway along that length; the total elongation of 50 mm gaugelength being 32%. For articles in which the cast metal component isbrittle, it is found that the bond is stronger than the component of thearticle of the cast metal. Thus, with hypoeutectic chromium white ironcast against and bonded to mild steel, bend tests showed fracture pathspassed through the white iron, and not the bond zone.

What is claimed is:
 1. A method of forming a composite article havingfirst and second metal components, wherein said first component is aferrous metal and said second component is a ferrous metal or cobaltbase alloy comprising the steps of:(a) applying a flux coating over asubstantially oxide-free bond surface of said first component; (b)preheating said first component in a mould in which said first componentis positioned to a preheat temperature of about 350° C. to about 800°C.; and (c) pouring a melt of said second metal to provide said secondcomponent, said melt being poured at a superheated temperature and suchthat said melt flows over said bond surface to thereby displace saidflux coating from said bond surface and wet said bond surface, saidsuperheat temperature being substantially in excess of said preheattemperature, whereby said melt raises the temperature of said bondsurface to achieve an initial temperature equilibrium between saidsurface and the melt, and a substantially instantaneous interfacetemperature therebetween which is at least equal to the liquidustemperature of the melt, such that on solidification of the melt a bondbetween the components is attained substantially in the absence offusion of said bond surface.
 2. A method as defined in claim 1, whereinsaid first component comprises a ferrous metal selected from mild seel,low alloy steels and stainless steels.
 3. A method as defined in claim1, wherein said second component is selected from the group consistingof white cast irons, stainless steel, and cobalt-base alloys.
 4. Amethod as defined in claim 3, wherein said first component is selectedfrom the group consisting of mild steels, alloy steels, includingstainless steels, and cast irons including chromium white cast iron, andwherein said second component is a white cast iron having from 2.0 to5.0 wt.% carbon and chromium up to 30 wt.%.
 5. A method as defined inclaim 4, wherein chromium is present in excess of 14 wt.%, such as from25 to 30 wt.%.
 6. A method as defined in claim 4, wherein said whitecast iron has a composition selected from the group consisting of:(a)2.4 to 3.6 wt.% carbon, 0.5 to 1.5 wt.% manganese, 1.0 wt.% maximumsilicon, 14 to 17 wt.% chromium and 1.5 to 3.5 wt.% molybdenum, thebalance apart from incidental impurities being iron; (b) 2.3 to 3.0 wt.%carbon, 0.5 to 1.5 wt.% manganese, 1.0 wt.% maximum silicon, 23 to 30wt.% chromium, and 1.5 wt.% maximum molybdenum, the balance apart fromincidental impurities being iron; (c) 2.5 to 4.5 wt.% carbon, 2.5 to 3.5wt.% manganese, 1.0 wt.% maximum silicon, 25 to 29 wt.% chromium, and0.5 to 1.5 wt.% molybdenum, the balance apart from incidental impuritiesbeing iron; (d) 4.0 to 5.0 wt.% carbon, 1.0 wt.% maximum manganese, 0.5to 1.5 wt.% silicon, 18 to 25 wt.% chromium, 5.0 to 7.0 wt.% molybdenum,0.5 to 1.5 wt.% vanadium, 5.0 to 10.0 wt.% niobium, and 1.0 to 5.0 wt.%tungsten, the balance apart from incidental impurities being iron; and(e) 3.5 to 4.5 wt.% carbon, 1.0 wt.% maximum manganese, 0.5 to 1.5 wt.%silicon, 23 to 30 wt.% chromium, 0.7 to 1.1 wt.% molybdenum, 0.3 to 0.5wt.% vanadium, 7.0 to 9.0 wt.% niobium, and 0.2 to 0.5 wt.% nickel, thebalance apart from incidental impurities being iron.
 7. A method asdefined in claim 3, wherein said first component is selected from thegroup consisting of mild steel and alloy steels including stainlesssteels and wherein said second component is an austenitic stainlesssteel having a composition selected from the group consisting of:(a)0.08 wt.% maximum carbon, 18 to 21 wt.% chromium, 10 to 12 wt.% nickel,2 to 3 wt.% molybdenum and, apart from incidental impurities, a balanceof iron; and (b) 0.08 wt.% maximum carbon, 18 to 21 wt.% chromium, 8 to11 wt.% nickel and, part from incidental impurities, a balance of iron.8. A method as defined in claim 3, wherein said first component isselected from the group consisting of mild steel and alloy steels, andwherein said second component is a cobalt-base alloy having (Co, Cr)₇ C₃carbides in an eutectic structure and a work hardenable matrix, obtainedwith a composition selected from the group consisting of:(a) 28 to 31wt.% chromium, 3.5 to 5.5 wt.% tungsten, a maximum of 3.0 wt.% for eachof iron and nickel, a maximum of 2.0 wt.% for each of manganese andsilicon, 1.5 wt.% maximum molybdenum, 0.9 to 1.4 wt.% carbon and, apartfrom incidental impurities, a balance of cobalt; and (b) substantially29 wt.% chromium, 6.3 wt.% tungsten, 2.9 wt.% iron, 9.0 wt.% nickel, 1.0wt.% carbon and, apart from incidental impurities, a balance of cobalt.9. A method as defined in claim 1 wherein said first component ispreheated at least in part by flame heating applied within the mouldcavity, and maintained until after pouring of the melt is complete. 10.A method as defined in claim 9, wherein said flame heating providesreducing conditions within the mould cavity at least until pouring ofthe melt is complete.
 11. A method as defined in claim 1, wherein saidfirst component is preheated at least in part by flame heating appliedthereto in a drag component of the mould, prior to positioning of a copeportion of the mould, and said flame heating is terminated prior topositioning of said cope portion and pouring of the metal.
 12. A methodas defined in claim 1 wherein said flux is applied to said firstcomponent as a slurry.
 13. A method as defined in claim 1 wherein saidflux is applied to said first component as a powder.
 14. A method asdefined in claim 1, wherein said flux acts both to prevent oxidation ofsaid surface of the first component and also to clean said surface ofany oxide contamination.
 15. A method as defined in claim 1, wherein themetal of the first component has a melting range which commences at atemperature equal to or in excess of the liquidus temperature of themelt.
 16. A method as defined in claim 1, wherein the metal of the firstcomponent has a melting range substantially the same as that of themetal for the melt providing the second component.